1. Field of Invention
This invention relates to an improved oil well casing centralizer. More particularly, but not by way of limitation, the invention pertains to a device for reducing the torque required to rotate a casing positioned in a borehole during primary cementing of the casing.
2. Description of Prior Art
In completing oil or gas wells, the borehole is usually lined with a steel pipe (known as a "casing" or a "liner") held in place by cement located in the annular area between the casing's outer surface and the borehole wall. Typically, centralizers are used to center the casing in the borehole during cementing. A centered casing ensures a cement column of substantially uniform thickness and reduces channeling of the cement (discussed below).
One type of centralizing device is a bow spring centralizer. A typical bow spring centralizer is comprised of a fixed sleeve, a slidable sleeve, and a plurality of longitudinal bow springs extending therebetween. The fixed and slidable sleeves circumscribe the casing while the flexible bow springs are bowed outwardly to contact the borehole wall. As is well known in the art, longitudinal movement of the slidable sleeve is used to vary the amount of curvature of the bow springs. The centralizer, thereby, is able to substantially center the casing in the borehole despite variations in borehole diameter. Once centered, the casing may be cemented in place by pumping a column of cement into the annular space between the casing and borehole wall.
One type of cementing employed in oil and gas well completion is primary cementing. Primary cementing occurs immediately after the casing is run into the borehole. Its purpose is to provide a protective sheath around the casing and to prevent production of undesired fluids from strata above or below the zone of interest.
A problem frequently encountered in primary cementing is channeling of the cement. Such channeling arises from the cement slurry's inability to completely displace the drilling mud which surrounds the casing. Typically, a cement slurry is pumped down through the casing and then up the annulus to displace the drilling mud from the casing/borehole annulus. Mud channels develop when portions of drilling mud are not displaced by the cement. This is sometimes caused by the casing not being well centered in the borehole. Under such circumstances, it is difficult for the cement to displace the drilling fluid from the narrow side of the casing/borehole annulus. The undisplaced drilling mud may then later be displaced by water (or gas) from a surrounding reservoir through one or more of the production perforations. The channel thereby causes the production of unwanted fluid. This channeling effect has been well documented in field and laboratory studies including "An Investigation of Oil Well Cementing", Drill. & Prod. Prac., API (1946) 76, Teplitz, A. J. and Hassebrock, W. E. and "Factors to be Considered in Obtaining Proper Cementing of Casing", Drill. & Prod. Prac., API (1948) 257, Howard, G. C. and Clark, J. B.
A second type of cementing is remedial or squeeze cementing, which is used to repair the primary cement sheath. Such cementing involves injecting cement behind the casing to recement channeled areas or to block off an uncemented zone. Eliminating or minimizing cement channeling facilitates primary cementing. Improved primary cementing in turn helps prevent lost hydrocarbons through interzonal flow, minimizes production of unwanted fluids, and reduces remedial cementing costs.
A variety of techniques for preventing or minimizing the channeling effect are known. These techniques generally focus on some method for improving the displacement of drilling mud by the cement slurry. "Displacement Mechanics in Primary Cementing" J. Pet. Tech. (Feb., 1967) 251, McLean, R. H. et al. identifies casing movement as one technique for improving mud displacement. Moreover, McLean recommends casing rotation as the preferred means of moving the casing. Subsequent studies confirming improvement in primary cementing from casing rotation include, "Liner Rotation While Cementing: An Operator's Experience South Texas", SPE Prod. Eng. (March, 1986) 153, Arceneaux, M. A., Smith, R. L. and "Rotation of a Long Liner in a Shallow Long-Reach Well", J. Pet. Tech. (April, 1989) 401, Gust, D. A., MaCDonald, R. R.
With a typical centralizer, a casing may rotate within the centralizer while bands or stop rings, rigidly attached to the casing's outside diameter, prevent the centralizer from moving longitudinally along the casing. High torque, however, is usually required to rotate a casing positioned by one or more centralizers in a borehole. The coefficient of friction produced by the contact of steel casing with a centralizer's steel sleeves necessarily requires significant torque at the surface to overcome the static and kinetic frictional forces. In some cases, the required torque at the surface may exceed the capacity of the drilling rig. In other cases, high-cost casing connections with high-torque capacity are required where casing rotation is desired and static and kinetic frictional forces are high. The proposed invention reduces the torque required to rotate a centralized casing during primary cementing which may facilitate rotation with available equipment and/or allow using lower-torque connections having lower cost.
Some previous patents have suggested a variety of means for reducing static and kinetic frictional forces in rotating various types of drilling apparatus. None of these patents, however, recognize the problems posed by small clearances between the casing and borehole wall, nonuniform wear caused by lateral loads on the bearing surfaces, or increased friction and deterioration of the bearing surfaces caused by particles and debris in the drilling fluid.
In European Patent 140311, E. O. Anders discloses an apparatus for reducing friction resisting rotation of a drill string in inclined well bores. The apparatus includes a rigid tubular body adapted to connect to a drill string and a sleeve of elastomeric material loosely mounted on the tubular body. The apparatus allows rotation of the drill string and tubular body relative to the elastomeric sleeve which contacts the borehole wall or casing. This rotation thereby occurs with less frictional resistance than would be produced by a drill string rotating against the borehole wall or casing in the absence of such an apparatus. Anders, however, does not suggest any means for adapting the elastomeric material to a bow spring type centralizer. Furthermore, in Anders' apparatus, the drill string rotates inside a stationary elastomer bearing. In deviated wells, this will result in uneven wear and therefore reduced bearing life.
The bow spring centralizer is preferable to the rigid centralizer described by Anders where the borehole diameter varies. Borehole diameter variations can arise where a tapered casing string is used, where casing wall thicknesses may vary for tensile or pressure designs purposes, or where borehole washout has occurred at deeper depths. Borehole washout is where the diameter of the borehole is greater than the diameter of the bit used to drill it. This is caused by the erosional effect of the circulating drilling fluid or by spalling or caving in of unstable formations. This phenomenon frequently occurs in well drilling and is well known to those skilled in the art. As described above, the bow spring centralizer can either contract or expand its outside diameter to adapt to a range of borehole diameters that may be found in a single borehole. A rigid centralizer, however, has a fixed outside diameter which must not exceed the minimum inner diameter of any previously-run casing.
If the elastomeric bearing surface is stationary, as in Anders' apparatus, it will wear unevenly in deviated wells where the side loads on the bearing act predominantly toward one side (usually the low side) of the hole. This will result in reduced bearing life, which becomes a greater concern if clearance considerations limit bearing thicknesses.
Consequently, a need exists for an improved bow spring type centralizer which reduces the torque required to rotate the casing positioned in a borehole during primary cementing. This improvement should be easily and economically incorporated into such a centralizer without reducing the centralizer's adaptability to various borehole diameters. Also, the improvement should facilitate extended rotation times by providing for even wear of the principal bearing surface.